In the frequency range of terahertz waves, many organic molecules in biological materials, medicines and, electronic materials have absorption peaks due to their structures and states. Furthermore, terahertz waves have high transmissivity to materials, such as paper, ceramics, resin, and cloth. In recent years, researches and developments have been performed on imaging technology and sensing technology making use of such characteristics of terahertz waves. The technology has been expected to be applied to, for example, a safe fluoroscopic inspection apparatus as an alternative to an X-ray apparatus and an in-line non-destructive inspection apparatus in a manufacturing process.
As a terahertz wave light source of a current injection type, a structure has been studied. The structure uses an electromagnetic wave gain based on intersubband transition of electrons in a semiconductor quantum well structure. Non-Patent Literature 1 (NPL 1) discusses an oscillator using a resonant tunneling diode (RTD) as a negative resistance device and a slot antenna as a resonator. The slot antenna in NPL 1 includes metallic patterns on a semiconductor substrate and the end portion of the slot has a capacitance. The oscillator of NPL 1 further includes a rectifying diode. Here, the rectifying diode serves as a stabilization circuit for suppressing parasitic oscillation posing a problem in an oscillator with a negative resistance device. The term “parasitic oscillation” refers to oscillation parasitically occurring in a band of low frequency waves different from desired frequency. Such parasitic oscillation causes a significant decrease in an oscillation output at the desired frequency. Thus, the presence of a stabilization circuit in an oscillator with a negative resistance device is very important.
When λosc is an oscillation wavelength of an oscillator, and ωosc is an oscillation frequency, in order to suppress the parasitic oscillation, an impedance on a power supply side for bias supply should be low in a frequency range of equal to or larger than direct current (DC) and smaller than ωosc. As a technique for achieving this purpose, a low impedance circuit serving as the stabilization circuit (e.g., a shunt rectifier diode) can be arranged at a position within λosc/4 as viewed from the RTD toward the power supply side. Thus, in an example of NPL 1, a rectifier diode is arranged in an integrated state as such a low impedance circuit at a position within λosc/4 as viewed from the RTD toward the side of the power supply.
On the other hand, Non-Patent Literature 2 (NPL 2) discusses a double-sided metal (DSM) waveguide structure as a low-loss terahertz waveguide in a quantum cascade laser (QCL). Here, if the DSM waveguide is used as a RTD resonator, frequency stabilization is important because the RTD has gain over a wide frequency range from DC to terahertz wave band while the QCL has gain over a narrow frequency range.